1. Field of the Invention
The present invention relates to facilitated transport membranes for olefin/paraffin separation, and more particularly to silver-containing polymer membranes with improved stability for olefin/paraffin separation and a method for preparing the same.
2. Description of the Prior Art
Olefins, basic raw materials for the chemical industry, are mainly obtained by cracking naphtha or natural gas as mixtures with saturated hydrocarbons with close volatilities, and thus olefins must be separated before they can be used.
Currently, separation of olefin/paraffin mixture is mainly carried out by highly energy-intensive low temperature distillation. As a result, a number of alternatives have been investigated including adsorption and membrane separation.
There was reported a method for separating olefins from paraffins by adsorption using silver or copper salts which reversibly react with olefins (see, Ind. Eng. Chem. Res. 2571 (1998)). However, this separation method using adsorption is disadvantageous because desorption process must be carried out after adsorption, thereby requiring a large equipment and space.
Recently, separation by facilitated transport membranes in which silver salts are used as carriers has attracted much interest because of the low energy consumption, compact apparatus, and simple operation. The basis for the separation is the ability of silver ions to interact reversibly with olefins by forming silver-olefin complexes. There have been many reports on the facilitated transport of olefins by using various membranes such as supported liquid membranes and ion-exchange membranes; however those membranes, as disclosed in U.S. Pat. No. 3,758,603, exhibit high olefin/paraffin selectivity only in the presence of water, requiring costly and undesirable humidification and dehumidification steps in the practical applications.
A solution to this problem is the development of facilitated transport polymer membranes. U.S. Pat. No. 5,670,051 discloses a facilitated transport polymer membrane in which silver salt present in polymer matrix interacts with olefins. According to this patent, separation of olefins from paraffins is conducted in the absence of water, so that loss of the silver salt caused by introduction of gas does not occur. Furthermore, since the polymer membrane can be produced so as to have a reduced thickness of several xcexcm or below, gas permeability can be significantly increased. In addition, much larger amounts of silver salts can be used in comparison with other types of membranes, and thus the separation efficiency can be significantly increased.
However, polymer membranes containing silver salts exhibit some disadvantages that prevent the commercialization of this separation process. One major drawback observed in the polymer membrane is the decrease in membrane performance with time, possibly due to the reduction of silver ions to silver particles by light or impurities.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a silver salt-containing facilitated transport membrane for olefin separation having improved stability and also a method for producing the same, which exhibits no deterioration in membrane performance even when it is operated for an extended period of time.
To accomplish this object, there is provided a facilitated transport membrane for olefin/paraffin separation, which comprises a polymer, a silver salt, and a phthalate compound represented by the following formula (1): 
wherein R denotes an alkyl group of 2 to 8 carbon atoms or a phenyl group.
In the facilitated transport polymer membranes according to the present invention, phthalates are able to bind to silver ions in a chelating mode, thereby improving the stability and performance of the membranes.
The membranes in the present invention comprise polymer matrix as solvent and a silver salt capable of reversibly interacting with olefins as transport carrier and solute.
The material chosen for the polymers should promote dissolution of the silver salt into the polymer matrix, and provide high mobility for complexed olefin molecules.
Based on these factors, preferred polymers are those that can form coordination bonds to silver ions by means of oxygen, nitrogen or sulfur electron-donating atom, known as a hetero-atom, in the polymer structure.
In other words, the polymer is used, which has a functional group, such as amide, ether, ester or carbonylic acid. Examples of this polymer include polyvinylpyrrolidone (PVP), poly(2-ethyl-2-oxazoline), polyvinylmethylketone, polyvinylformal, polyvinylacetate, cellulose acetate (CA), cellulose acetate butyrate (CAB), polyacrylate, polymethylmethacrylate (PMMA), polyacrylic acid and the like.
In the polymer membranes according to the present invention, the molar ratio of silver salt/monomer unit of polymer ranges from 0.5-3, and preferably 1-2. Examples of the silver salt, which can be used in the practice of the present invention, include AgBF4, AgPF6, AgSO3CF3, AgClO4, AgSbF6 and the like. In addition, other silver salts, which can chemically bind to the functional group of the polymer, may also be used.
The phthalate compound is contained at the amount of 0.05 to 10% by weight, and preferably 1 to 5% by weight, relative to the weight of the polymer. If the phthalate compound content is below 0.05% by weight, it is disadvantageous in that an improvement of performance and stability of the separation membrane is insufficient. On the other hand, if the phthalate content exceeds 10% by weight, it is also disadvantageous in that the permeance of olefins is significantly reduced.
Furthermore, the facilitated transport membrane for olefin/ paraffin separation is produced by a method which comprises the steps of: successively dissolving a polymer, a silver salt and a phthalate compound in a solvent to form a homogeneous solution; coating the solution on a support; and drying the coated support at a condition free of light and oxygen.
The solvent that is used in the method of the present invention should dissolve all of the polymer, the silver salt and the phthalate compound and should be completely removed at the drying step after coated on a support or glass plate, etc. Examples of this solvent include alcohols of 1 to 4 carbon atoms, and tetrahydrofuran.
The solvent or solvents used to prepare the coating solution should not attack the supporting substrate. It is preferred to use a supporting substrate, which is microporous and thus exhibits an excellent permeability.
The facilitated transport membrane may also be produced by a method where the solution is coated on the glass plate without using the support, dried and then removed from the glass plate. In any case where the solution is coated on the support or on the glass plate, the thickness of the polymer film is typically several xcexcm although it can vary if necessary.
Separation of olefin/paraffin using the polymer membrane produced by the above method is carried out at a temperature and pressure at which permeates can be present in a gas state. Separation of olefin/paraffin mixtures, which have low molecular weights and are gaseous at room temperature, can be carried out at room temperature as in cases of ethane/ethane, propane/propane, and 1-butene/butane mixtures. However, separation of olefin/paraffin mixtures, which have high molecular weights and thus are present as liquid at room temperature, is carried out at a temperature higher than the boiling point of the mixture by more than 10xc2x0 C.
Separation of the olefin/paraffin mixture using the facilitated transport membrane is achieved by diffusion of gas mixture being introduced and transport of complexes formed between carriers and olefins being introduced. As a result, in view of such two effects, a pressure difference across the membrane (permeate pressure) is preferably in the range of 1 to 5 atm.
The present invention will hereinafter be described in further detail by examples. It should however be borne in mind that the present invention is not limited to or by the examples.